TOKYO - Japan rejected the outcome of the Crimean referendum on Monday, saying it was "illegal" despite an overwhelming majority vote in favor of joining Russia.

"The referendum is not legally effective, and Japan does not accept the outcome," Japan's Chief Cabinet Secretary Yoshida Suga told reporters earlier in the day.

The plebiscite, in which 95.5 percent of voters were in favor of rejoining Russia as shown by the preliminary results, "goes against Ukraine's Constitution which stipulates that any potential change to its territory should be put to a national vote," said the top Japanese government spokesman.

"We will strongly urge Russia to abide fully by international law, respect the sovereignty and territorial integration of Ukraine, and not to annex Crimea," he said.

Some senior officials in Tokyo vows that Japan would work with other Group of Seven countries in handling the Ukraine crisis. But analysts said considering ongoing territorial negotiations between Japan and Russia, the former now faces a dilemma over how to respond to the escalation of the crisis in Ukraine.

"We want to respond appropriately after hearing the debates in related parties such as the European Union, Russia's response and the situation in Ukraine," Japan's Foreign Minister Fumio Kishida told reporters separately.

Crimea, historically part of the Russian Federation, was transferred to Ukraine in May 1954, then a republic of the Soviet Union.